In many areas of technology, an object placed on a table that is movable in at least two directions must be very accurately positioned together with the table relative to a tool. In this connection, interferometers are frequently used as position-measuring devices. However, due to variations in the refractive index of the air traversed by the laser light, such interferometers are unable to measure with sufficient accuracy and, therefore, position-measuring devices based on the scanning of scales are being increasingly used for such applications. Such measuring devices are less prone to failures resulting from environmental conditions because the distance traveled by the light for scanning the scales is only a few millimeters, which is significantly less than when using interferometers, where light paths of several decimeters must be reckoned with.
International Patent Publication WO 2007/034379 A2 describes an optical position-measuring device where a table is positionable relative to an underlying stationary base in two orthogonal directions parallel to the underlying base. For this purpose, transmissive scales are attached laterally to the edges of the table. Light from scanning heads passes through the transmissive scales and impinges on reflective scales located above the table in stationary relationship with respect to a tool. From there, the light is reflected and passed through the transmissive scales back to the scanning heads. Each scanning head scans a transmissive scale and a reflective scale arranged at a right angle thereto. Owing to the crossed arrangement of the two scales, a scanning head is cable of measuring the position of the table in one direction independently of the position of the table in the respective other direction. With three such measuring devices, it is possible to measure the two linear directions (X and Y) extending in the plane of the table and the rotation (rZ) about an axis perpendicular to the plane of the table. This publication further explains that the crossed scales may have periodic structures in both linear measurement directions, so that measurement in both measurement directions is possible using suitable sensors at a point of intersection of two scales.
European Patent Application EP 2450673 A2, which also deals with such measuring devices, further describes ways of measuring also the linear direction (Z) normal to the plane of the table using a measuring device for a direction of movement (X or Y) of the table. With three such measuring devices, it is possible to measure all degrees of freedom of the table; i.e., the translation directions X, Y and Z as well as the rotations about the translation axes; i.e., rX, rY and rZ.